Vacuum electric furnace

ABSTRACT

An electric furnace is disclosed for operation at vacuum with or without inert gas with an external water cooled enclosure having an access door and within which enclosure large areas low mass quick heating resistance elements are provided for the back and front, sides, top and bottom with independent control of the heating elements, programmed if desired, the heating units being shielded to minimize heat loss outside the enclosure by low mass shielding elements including a baffle in one wall which permits rapid gas exit for fast pump down to obtain a fast cycle time while limiting external thermal radiation losses from the shielded area.

if g units being shielded to minimize 1 June 5, 1973 .263/40RX ...l3/31 X Kremers..........................t........13/25 ABSTRACT An electric furnace is disclosed for Operation at vacuum with or without inert gas with an external water cooled enclosure having an access doorand low mass quick-,1", heating resistance elements are provided for the back, pendent programmed e wall which permits own to obtain a fast cycle time while limiting external thermal radiation 3,295,843 1/1967 McMaster et al. 3,155,758 11/1964 Hil]........ 2,640,861 6/1953 Primary ExaminerRoy N. Envall, Jr. Attorney-Zachary T. Wobensmitli, I1

within which enclosure large areas and front, sides, top and bottom with inde control of the heating elements, desired, the heatin heat loss outside the enclosure by low mass shielding elements including a baffle in on rapid gas exit for fast pump d losses from the shielded area.

12 Claims, 15 Drawing Figures VACUUM ELECTRIC FURNACE [75] Inventors: Benjamin A. Kreider, Richboro; William R. Jones, Chalfont; William J. Metalsky, Warminster; Thomson B. Gibb, Warrington, all of Pa.

Abar Corporation, Feasterv ille, Pa. Filed: Dec. 9, 1971 Appl. No.: 206,383

ULs.cl...........................13/25,13/211,219/343,

Field of Search..................... 347, 219/350, 390, 391, 395,405; 13/20, 25

References Cited UNITED STATES PATENTS 12/1964 Potenzo et al... .....,..............13/31 X 7/1966 Hill........ ...13/31 X United States Patent 1 Kreider et a1.

[73] Assignee:

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PATENTED JUN 5 I 75 SHEET l UF 7 PATENTEUJJH 5mm 37 7,5513

SHEET a [If 7 1 VACUUM ELECTRIC FURNACE BACKGROUND OF THE INVENTION This invention relates to electric furnaces for vacuum operation.

DESCRIPTION OF THE PRIOR ART It has heretofore been proposed to provide vacuum electric furnaces with shielding, conventional designs having a plurality of and usually four to eight, parallel radiation shields, which include at least seven surfaces cold enough for condensation of condensible materials. These condensation surfaces are the inner wall of the tank and the front and back of inner shields. Condensation does not usually occur on the innermost shield because of its higher operating temperature.

It has also heretofore been the accepted practice to use heavy bars or rods for constructing the heating elements with such mass that slow heating and slow cooling occurs. Such bars also do not, because of their spacing and limited area, provide uniformity of heating of the work space.

It has heretofore been the accepted practice to employ a large round of cylindrical vacuum chamber with excessive unutilized volume.

In the heating chambers heretofore employed in vacuum furnaces it has been common practice for the gases to escape by diffusion through the cracks and gaps in the shielding, this type of circulation, however, being slow.

The furnace of the present invention departs from prior practice in substantial respects so as to provide a faster operating cycle than heretofore, the heating elements and shielding elements having relatively low massand thus low thermal inertia thereby permitting quick heating and cooling.

SUMMARY OF THE INVENTION In accordance with the invention a vacuum electric furnace is provided in which the vacuum chamber is preferably rectangular in cross section to eliminate wasted interior space, preferably has a full front opening door for access to the interior, the chamber being water cooled, and valved for high speed vacuum pumping, the six walls having thin low mass heating elements substantially covering their surfaces, the temperatures of these heating elements being closely controlled, with programming of the energization of the heating elements, and with thin low mass shields surrounding the hot zone and arranged for gas escape without radiation escape to avoid condensation on the shields. The thermal losses inherent in a furnace having a single shield, as described,.is compensated for by a faster response to heating and cooling due to low thermal inertia of the shielding and heating elements.

BRIEF DESCRIPTION OF THE DRAWINGS The nature and characteristic features of the invention will be more readily understood from the following description taken in connection with the accompanying drawings forming part hereof, in which:

FIG. 1 is a front elevational view of a furnace in accordance with the invention;

FIG. 2 is a front elevational view of the interior of the furnace with the door removed;

FIG. 3 is a horizontal sectional view taken approximately on the line 3-3 of FIG. 1;

FIG. 4 is a vertical sectional view taken approximately on the line 4-4 of FIG. 1;

FIG. 5 is a vertical sectional view, enlarged, taken approximately on the line 5-5 of FIG. 2;

FIG. 6 is a fragmentary sectional view, enlarged, taken at the area 6 of FIG. 2;

FIG. 7 is a fragmentary sectional view, enlarged, taken at the area 7 of FIG. 2 and illustrating one of the terminals;

FIG. 8 is a fragmentary sectional view, enlarged, taken at the area 8 of FIG. 2 and illustrating one of the work support pins;

FIG. 9 is a vertical sectional view, enlarged, taken approximately on the line 9-9 of FIG. 2;

FIG. 10 is a vertical sectional view, enlarged, taken approximately on the line 10-10 of FIG. 2 and showing one of the bottom heating elements and its electrical terminal connection;

FIG. 11 is an exploded perspective view of the structure of FIG. 10;

FIG. 12 is a plan view of the bottom shield;

FIG..13 isan end view of the shield shown in FIG. 12;

FIG. 14 is a longitudinal vertical sectional view taken approximately on the line 14-14 of FIG. 12; and

FIG. 15 is a schematic wiring diagram of the furnace power control.

It should, of course, be understood that the description and drawings herein are illustrative merely and that various modifications and changes can be made in the structure disclosed without departing from the spirit of the invention.

Like numerals refer to like parts'throughout the several views.

DESCRIPTION OF THE PREFERRED EMBODIMENT,

. horizontal top and bottom walls 21 and 22, spaced parallel vertical side walls 23 and 24, and a vertical rear wall 25 connecting the top, bottom and sidewalls 21,

22, 23 and 24 in vacuum tight relation.

The'walls of the tank 20 are of sufficient thickness to withstand the forces to which they are subjected under vacuum and when internally heated. The tank 20 has a front vertical peripheral flange 26 formed thereon.

A vertical front door 27 is'provided, hingedly carried on brackets 28 on one of the side walls, such a side wall 23, with hinge pins 29 engaged by hinged brackets 30 on the door 27, and held in closed position by a clamp 31. Shims 32 may be employed on the hinge pins 29.

Any suitable packing or gasket 33, such as an O-ring,

can be provided to prevent leakage when the is in closed position on the tank 20.

The walls 21', 22, 23, 24 and, 25 and the door 27 are preferably cooled by tubing secured thereto on the exterior and through which liquid for cooling is circulated. Such cooling structure is well known in the electric furnace art and the tubing and fluid circulating and control system have been omitted in the interest of clarity.

The tank 20 can be supported in any desired manner in spaced relation to the floor.

The bottom wall 22, at a plurality of spaced locations, is provided with vertically extending heat shield mounting pins 35 (see FIG. 6) with lower surrounding door 27 enlargements 36 having upper and lower retaining washers 37 and 38 and retainer wires 39 penetrating the pins 35 for retention.

A heat radiation shield 40 composed of a single thickness flat plate portion of reflective material and preferably of stainless steel 0.015 inches thick has spaced longitudinal stiffening ridges 42 thereon and transverse box stiffeners 43 secured thereto on their .undersides. The upper portions of the mounting pins 35 extend upwardly through openings 44 and 45 in which the washers 37 are disposed, the openings 45 being elongated to accommodate expansion and contraction. The lower faces of the box stiffeners 43 rest on the washers 38.

The shields 40 have side rims 46 with outwardly extending flanges 47 for overlapping relation with other flanges to reduce reflective leakage at their margins, as hereinafter explained.

The bottom wall 22 (see FIG. 7) also has a plurality of inserts 50 with openings 51 therethrough for the reception of heating element mounting pins 52 which provide exteriorly accessible terminals. The pins 52 at their outer ends are seated in contact pin blocks 53 which are mounted on the inserts 50 by bolts 54 in threaded engagement therein. The blocks 53 are electrically insulated from the inserts 50 by insulating packing gaskets 55 which also prevent vacuum leakage at these locations. The block 53 in FIG. 7 is shown as having an electrical conductor 160 mounted on a stud 74 and held by a nut 75 for supplying electrical energy for heating the heating element.

The bolts 54 are electrically insulated from the inserts 50 and blocks 53 by cylindrical gaskets 56, circular gaskets 57 and washers 58.

The heating element mounting pins 52 extend through clearance openings 59 in the heat shield plate 40. The inner or upper ends of the pins 52 are slotted for the reception of brackets 60. The brackets 60 are held in place by bolts 61 which also carry brackets 62 to which reflective shields 63 are secured to shield the clearance openings 59 against radiation leakage.

The heating elements on the bottom wall 22 comprise a plurality of thin flat strips 66 of suitable resistance material, preferably of sheet nickel or nickel alloy, four strips being shown, and which may be 0.005 inches by 2% inches.

The combined exposed area of the strips 66 facing the working space in the furnace and in which the work pieces are disposed is preferably in a range from percent to 80 percent of the interior surface of the bottom wall 22. A smaller area of heating surface gives rise to hot spots in the furnace and lack of uniformity of heat application to the work piece and variable temperature gradient where uniformity is desired.

The heating elements as described provide for rapid transfer of energy to the work with uniform heating. The high resistance of the relatively thin sheet nickel elements enables rapid heatingand fast thermal response for dynamic temperature control at low temperatures of the order of 1000 degrees F, and permit obtaining furnace temperatures, if desired, of the order of 2,l50 F.

The same area relationship of heating elements and shielding elements is preferably maintained on the other walls 23, 24, 25 and 21 and the door 27.

In a specific embodiment an exposed area of heating element to wall area of 60 percent has been found to be quite satisfactory.

The first strip 66 is secured at one end to the bracket 60 by bolts 67 which extend through clamping plates 68 and at its other end extends between upper and lower supporting plates 69 carried on a supporting pin 70 (see FIG. 9). The supporting plates 69 (see FIGS. 3 and 9) are held in engagement with the ends of the first and second strips 66 by bolts 71 and retainer wire 72.

The supporting pin 70 is supported by the plate 40 and a transverse box stiffener 80, a similar box stiffener being provided near the other end of the plate 41.

The box stiffener 80 has a flange 82 at each end (see FIG. 14) to space it upwardly from the floor 22.

In order to locate the supporting plates 69 with respect to the shield plate 40, and electrically insulate the strips 66 therefrom (see FIG. 9) an inner insulating sleeve 84 is provided surrounding the pin 70, engaged at the top with an upper disc 85 and at the bottom with a lower disc 86, the discs 85 and 86 being limited by retainer wires 87. Outer insulating sleeves 88 and 89, are interposed respectively between the disc 85 and the sheet 40 and between the bottom of the box stiffener 80 and the disc 86.

The insulating sleeve is accommodated in an opening 90 in the sheet 40. V

The second strip 66 and the contiguous end of the third strip 66 are similarly supported, and insulated, the opening 90 in the sheet 40 for the pin 70 at the right being shown on FIG. 12.

The other end of the third strip 66 and the contiguous end of the fourth strip 66 are supported and insulated in a similar manner with the second end of the fourth strip 66 having a terminal connection as previously described and illustrated in FIG. 7.

The strips 66 are preferably supported and electrically insulated from the wall 22 intermediate their ends (see FIGS. 10 and 11) by rectangular supporting rods which have an inner insulating sleeve 96 and an outer insulating sleeve therearound. The sleeves 96 and bear on a transverse arm 97 held together by retainer wires 98. The upper ends of the rods 95 have a cross arm 99 secured thereto by fasteners 100, such as rivets. The cross arm 99 is held in spaced relation to the shield plate 40 by the outer upper insulating sleeve 101. The plate 40 has box stiffeners 102 through which the rods 95 and sleeves 96 extend, the plate 41 having openings 103 for this purpose.

The box stiffeners 102 have a spacing flange 104 similar to flange 82 (see FIG. 14).

A lower insulating sleeve 110 surrounding the sleeve 96 is interposed between the bottom of the box stiffener 102 and the transverse arm 97.

The cross arm 99 has clamping rods 105 thereabove, and retainer wires 106 extending through opening 107 in the arm 99, through longitudinally elongated openings 108 in the strips 66 and into notches 109 in the rods 105 (see FIG. 11). The openings 108 accomodate expansion and contraction of the strips 66 upon change of temperature.

In order to support the work and/or work supports within the furnace the bottom wall 22 (see FIG. 8) at the desired locations has vertical work pins carried in sockets 1 16 on horizontal base plates 1 17. The plates 117 are held in place, with washers 118 therebelow, by

studs 119 welded to the bottom wall 22 and extending through the plate 117, and having nuts 120 and washers 121 thereon.

The work pins 115 extend through openings 1 22 in shields 40a and 40b, similar to the heat shield 40, with vertical longitudinal stiffening ridges 42a and 42b, transverse box stiffeners similar to the box stiffeners 43, transverse box stiffeners similar to the box stiffeners 102 and upper and lower transverse box stiffeners similar to the box stiffeners 80.

The heat shields 40a and 40b are held with respect to the side walls 23 and 24 in a manner similar to that previously described for the heat shield 40' by mounting pins 35a and 35b.

The side walls 23 and 24'also have heating element mounting and terminal pins 520 and 52b, similar to the mounting pins 52, extending through the heat insulating shields 40a and 40b, for supporting heating element strips 66a and 66b, similar to the strips 66, with supporting pins 70a and 70b for the intermediate ends of the strips 66a and 66b, and intermediate supporting rods 95a and 95b, and associatedstructure, similar to the rods 95, for supporting the strips 66a and 66b intermediate their ends.

The top wall'2l has a heat radiation shield 40c, similar to the heat shield 40, with vertical longitudinal stiffening ridges 42c, and transverse stiffeners similar to the box stiffeners 43 and 80, and a box stiffener 102.

The heat shield 40c is held with respect to the top wall 21 in a manner similar to that previously described for the heat shield 40 by mounting pins 35c.

The top wall has heating element mounting and terminal pins 52c, similar to the mounting pin 52 extending through the heat insulating shield for supporting heating elements strips 660 with supporting pins 700 for the intermediate ends of the strip 660, intermediate supporting rods 95c and associated structure for supporting the strips 660 intermediate their ends.

The top wall 21 is also provided with a central 'gas outlet opening for cooling gas discharge, connected to a valve controlled fluid connection 133 and below the top wall 21 duct 135 communicating therewith is secured to the shield 40a in amanner similar to the duct 125 and with a baffle l27c similar to the baffle 127 (see FIGS. 2 and 4).

' The door 27 on the interior thereof, has a heat radiation shield 40d, similar to the heat shield 40, but made in three vertical sections with overlapped joints 137 to accommodate expansion, with ribs 42d, box stiffeners similar to the box stiffeners 43 and 80, and with a central'box stiffener 102d corresponding to the box stiffener'l02.

The heat shield 40d is supported by pins 35d similar to the pin 35.

The-door 27 has heating element mounting pins 52 extending through the shield 40d for supporting heating element strips 66d similar to the strips 66 but ofa width 6 of the order of five inches, with end supporting pins d, and an intermediate supporting rod 95d.

The rear wall 25, on the interior thereof, has a heat radiation shield 402, similar to the shields 40 and 40d, and like the heat shield 40d made in three vertical sections with overlapped joints 137 to accommodate expansion, and with horizontal box stiffeners e and 102e, similar to. the box stiffeners 80 and 102.

i The heat shield 40e is held with respect to the rear wall in a manner similar to that previously described by mounting and terminal pins 35c.

The rear wall 25 has a heating element mounting pin 52e extending through the shield 40e for supporting heating element strips 66e, similar to the strips 66 but with widths corresponding to the widths of the strips 66d, and with supporting pins 70e and an intermediate supporting rod e for securement of rods 105a.

The rear wall 25 has an opening aligned with an opening 141 in the rear baffle 40e for the reception of a baffle 142 which is optically dense but has very low resistance to gas flow.

The baffle 142, as shown in FIG. 5, includes a plurality of superposed horizontally disposed inverted V- shaped baffle walls 143, the front margins of each wall 143 extending downwardly at least as far as the upper margin of the wall 143 of the baffle element therebelow. The baffle 142- is mounted on rods 144 which are secured in forwardly extending clips 146 carried by the rear wall 25 (see FIG. 2).

The baffle walls 143 are spaced apart for gas movement through the baffle 142 but by reason of their overlap interpose a barrier to thermal radiation from the interior of the tank 20.

It will be noted in FIG. 3 that the heat shields 40a and 40e, 40e and 40b, 40b and 40d, and 40d and 40a have their vertical rims and flanges 47 in overlapped relation and in FIG. 4 that the heat shields 40 and 40d, 40d and 400, 40c and 40e, and 40e and 40 havetheir horizontal rims and flanges 47 in overlapped relation to block the escape of thermal radiation from the interior.

A valve controlled fluid exhaust connection connects the interior of the tank 20 to an exhausting system of any desired type to quickly achieve and rn'aintain the desired vacuum within the tank 20. Such system can include motor driven roughing and holding pumps and a diffusion pump if desired.

Referring now to FIG. '15, a schematic wiring diagram is shown which for furnace power control includes a connection through input leads 150, 151 and 152, from a source of three phase current and through a circuit breaker CB1.

The power supply for the heating elements can be obtained in any desired manner such as by the use of variable reactance transformers or saturable core reactors, the latter being hereinafter specifically referred to.

One phase is. preferably connected by conductors 153, 153a and 154 to the primary winding of a-step down transformer T1 with the conductor 153a connected through a saturable core reactor SR1 to vary the output of the transformer T1 as required.

The secondary of the transformer T1 is connected by conductors 155 and 156 to the mounting pins 52'atthe opposite ends of the door mounted heating element strips 66d and the rear wall mounted heating element strips 66e so that these strips act together.

Another phase is preferably connected by conductors 154, 1540 and 157 to the primary winding ofa step down transformer T2 with the conductor 154 connected through a saturable core reactor SR2 to vary the output of the transformer T2 as required.

The secondary of the transformer T2 is connected by conductors 159 and 160 to the mounting pins 52 at the opposite ends of the top wall mounted heating element strips 66c and the bottom wall mounted heating element strips 66 so that these strips act together.

The remaining phase is preferably connected by conductor 153 and 157, 157a to the primary winding of a step down transformer T3, with the conductor 157, 157a connected through a saturable core reactor SR3 to vary the output of the transformer T3 as required.

The secondary of the transformer T3 is connected by conductors 163 and 164 to the mounting pin 52 at the opposite ends of the side wall mounted heating element strip 66a and 66b of the side walls 23 and 24 so that these strips act together.

A thermocouple TC is provided in the interior of the tank and is preferably secured to one of the rear wall mounted heating element strips 66e so as to see the working space, and about half the distance between the top and bottom of the strip 66e, and at a location in from the interior of a side wall, such as side wall 24. A suitable location is on the second strip 66e, from the right at the location marked TC. The leads TCL from the thermocouple TC can extend rearwardly in the space between the first and second strips 66e from the right, rearwardly and upwardly, and through a suitable sealing glad in the top wall 21.

The thermocouple leads TCLl and TCL2 preferably are connected to a temperature recorder and controller RT for recording the'actual furnace temperature and for providing a process variable voltage and a programmer P can be utilized providing a set point voltage for making available through conductor 165 with the signal from the temperature recorder RT through conductor 166 as inputs to a controller C, having proportional band, rate response and reset.

The controller C provides a control signal through conductors 167 and 168 to an SCR pilot amplifier PA. From the power amplifier PA conductor 169 and 170 are connected to a direct current power supply DCPSl from which a variable d-c voltage (rectified a-c signal) 'is made effective through conductors 171 and 172 to the saturable core reactor SR1 to control the output of the transformer T1.

The controller C also provides a signal through conductors 174 and 175 through an autotransformer trimmer TRIM 1, for adjustment, to a direct current power supply DCPS2 from which a variable d-c voltage is made effective through conductors 176 and 177 to the saturable core reactor SR2 to control the output of the transformer T2.

The controller C also provides a signal through conductor 180 and 181, through an autotransformer trimmer TRIM 2, for adjustment, to a direct current power supply DCPS3 from which a variable d-c voltage is made effective through conductors 182 and 183 to the saturable core reactor SR3 to control the output of the transformer T3.

The power input to the respective heating element argon, the furnace opened and the work pieces removed. v

The low mass of the heating strips 66 to 66, and of the radiation shields 40 to 40c, permits rapid cooling. Where the furnace is reloaded, the low mass of the heating elements 66 to 66e permits rapid heating with evacuation effected with minimized loss through the baffle 142 to which the entire interior space within the radiation shields 40 to 40e is accessible. No withdrawal through relatively impervious walls is required.

We claim: 7 1. A vacuum electric furnace comprising a closed tank having a plurality of enclosing walls and a removable closure wall providing in the interior a working space,

each of the said walls having a self-sustaining reflective radiation shield member spaced inwardly therefrom,

means carried by said walls for supporting said shield members,

each of said shield members having spaced inwardly therefrom a resistance heating element with terminal ends and comprising flat strips of metal covering a substantial portion of the corresponding shield member, and

means carried by each of said shield members for supporting said resistance heating elements at a plurality of spaced locations.

2. A vacuum electric furnace as defined in claim 1 in which the exposed area of said heating elements is in the range of from 40 to percent of the exposed area of the radiation shield members.

3. A vacuum electric furnace as defined in claim 1 in which said heating elements each comprises a plurality of parallel strips disposed in a plane bounding the working space in the furnace.

4. A vacuum electric furnace as defined in claim 1 in which each of said shield members for each wall comprises a single thickness of reflective metal.

5. A vacuum electric furnace as defined in claim 1 in which means is provided for controlling the energy supplied to said heating elements, and

said meansincludes temperature responsive sensing means in the interior of said tank.

6. A vacuum electric furnace as defined in which said temperature responsive sensing means has a portion secured on the inward facingportion of one of the strips.

7. A vacuum electric furnace as defined in claim 1 in which v the marginal edges of each of said shield members are in meeting relation to the marginal edges of contiguous shield members.

8. A vacuum electric furnace comprisingv a closed tank having a plurality of enclosing walls and a removable closure wall providing in the interior a working space,

each of said walls having a radiation shield member spaced inwardly therefor,

each ofsaid shield members having spaced inwardly therefrom and carried thereby a resistance heating element with terminal ends and comprising flat claim 5 in strips of metal covering a substantial portion of the corresponding shield member, and the shield member of one of said walls having an opening therethrough closed by an optically dense fluid flow baffle for gas withdrawal therethrough. 9. A vacuum electric furnace as defined in claim 8 in which said baffle member includes a plurality of parallel walls angularly disposed with respect to the contiguous shield member to permit rapid discharge of gas therethrough and with their respective edges disposed to'block passage of thermal radiation through said baffle. 10. A vacuum electric furnace as defined in claim 1 in which 3 said enclosing walls comprise opposite parallel vertical side walls, horizontal top and bottom walls, a

vertical rear wall and said removable closure wall is a front wall.

11. In combination a flat radiation shield member ofa single thickness of reflective metal for attachment to a furnace wall, and

a resistance heating element in spaced relation to and secured to said shield member comprising a plurality of flat parallel strips of metal with their edges in closely spaced relation,

said shield member being self-sustaining and having stiffening portions extending between marginal edges thereof.

12. The combination defined in claim 11 in which each of the heating strips is of a single thickness of metal. 

1. A vacuum electric furnace comprising a closed tank having a plurality of enclosing walls and a removable closure wall providing in the interior a working space, each of the said walls having a self-sustaining reflective radiation shield member spaced inwardly therefrom, means carried by said walls for supporting said shield members, each of said shield members having spaced inwardly therefrom a resistance heating element with terminal ends and comprising flat strips of metal covering a substantial portion of the corresponding shield member, and means carried by each of said shield members for supporting said resistance heating elements at a plurality of spaced locations.
 2. A vacuum electric furnace as defined in claim 1 in which the exposed area of said heating elements is in the range of from 40 to 80 percent of the exposed area of the radiation shield members.
 3. A vacuum electric furnace as defined in claim 1 in which said heating elements each comprises a plurality of parallel strips disposed in a plane bounding the working space in the furnace.
 4. A vacuum electric furnace as defined in claim 1 in which each of said shield members for each wall comprises a single thickness of reflective metal.
 5. A vacuum electric furnace as defined in claim 1 in which means is provided for controlling the energy supplied to said heating elements, and said means includes temperature responsive sensing means in the interior of said tank.
 6. A vacuum electric furnace as defined in claim 5 in which said temperature responsive sensing means has a portion secured on the inward facing portion of one of the strips.
 7. A vacuum electric furnace as defined in claim 1 in which the marginal edges of each of said shield members are in meeting relation to the marginal edges of contiguous shield members.
 8. A vacuum electric furnace comprising a closed tank having a plurality of enclosing walls and a removable closure wall providing in the interior a working space, each of said walls having a radiation shield member spaced inwardly therefor, each of said shield members having spaced inwardly therefrom and carried thereby a resistance heating element with terminal ends and comprising flat strips of metal covering a substantial portion of the corresponding shield member, and the shield member of one of said walls having an opening therethrough closed by an optically dense fluid flow baffle for gas withdrawal therethrOugh.
 9. A vacuum electric furnace as defined in claim 8 in which said baffle member includes a plurality of parallel walls angularly disposed with respect to the contiguous shield member to permit rapid discharge of gas therethrough and with their respective edges disposed to block passage of thermal radiation through said baffle.
 10. A vacuum electric furnace as defined in claim 1 in which said enclosing walls comprise opposite parallel vertical side walls, horizontal top and bottom walls, a vertical rear wall and said removable closure wall is a front wall.
 11. In combination a flat radiation shield member of a single thickness of reflective metal for attachment to a furnace wall, and a resistance heating element in spaced relation to and secured to said shield member comprising a plurality of flat parallel strips of metal with their edges in closely spaced relation, said shield member being self-sustaining and having stiffening portions extending between marginal edges thereof.
 12. The combination defined in claim 11 in which each of the heating strips is of a single thickness of metal. 